Global energy consumption continues to rapidly rise with the majority of energy production coming from fossil fuels. However, continued fossil fuel dependency raises many concerns for the future: for example, environmental sustainability. Sustaining high levels of global energy consumption will require alternative fuel sources and more efficient use of existing fuels. One area of efficiency that can be immediately addressed is the recovery of waste heat.
More than 60% of energy generated in the United States is lost as waste heat which can be partially recovered using solid-state thermoelectric generators, devices that convert thermal gradients into electrical energy (Seebeck effect).
Thermoelectric effects have been known since the 1800's for their application in power generation and refrigeration. The 1950's brought renewed interest in thermoelectric devices with successful research focused on the bismuth telluride (Bi2Te3) materials system.
However, applications for bismuth telluride thermoelectric devices were restricted due to poor cooling efficiencies. The mid 1990's saw an increase in theoretical work by different research groups promoting the application of nanostructured materials systems with higher efficiency than bulk thermoelectric materials. The nanostructured thermoelectric materials may exhibit ballistic electron transport, which can yield a high power factor (S2σ), and boundary or interface scattering of phonons reduces the thermal conductivity (κT). Theoretical work carried by Mahan et al. and Shakouri et al. shows that the cross-plane ballistic transport in metal/semiconductor superlattices can potentially be used for thermoelectrics to achieve a thermoelectric figure of merit (ZT) of around 4 to 5.
Metal/semiconductor superlattices with cross-plane transport offer a novel approach towards improving the thermoelectric figure of merit (ZT). The thermoelectric device performance and efficiency for generation is given by a dimensionless figure of merit, ZT. The figure of merit ZT, is given by;
      ZT    =                                        S            2                    ⁢          σ                                      K            e                    +                      K            l                              ⁢      T        ,where S is the Seebeck coefficient, σ is electrical conductivity, κ is the thermal conductivity, T is the absolute temperature (K). The promise of enhancement in (S2σ) is possible by engineering the hetero structures barrier height and moreover, cross-plane phonon scattering possibly helps in reducing lattice contribution to the thermal conductivity.
The existing thermoelectric devices for high temperature applications are restricted because of their low melting or decomposition temperatures, scarce and toxic component elements such as Bi2Te3, CoSb3 and PbTe. Oxides thermal and chemical stability at elevated operating temperature, naturally abundant, nontoxic and low production costs make them an attractive potential candidate for TE devices. Oxides have been previously avoided for TE devices due to strong ionic behavior, narrow conducting bandwidths from weak orbital overlap leading to localized electrons with low carrier mobilities. However, conventional thoughts on oxides changed when large power factors were observed by Terasaki et al. in the magnetic layered cobalt oxide material, NaxCo2O4. The power factor is comparable to Bi2Te3, but the mobility is one order of magnitude lower, suggesting that a low mobility conductor can also be an efficient thermoelectric material. Later, Wang et al. suggested that the reason for the large power factor in NaxCo2O4 is due to its anti-ferromagnetic behavior at room temperature. The spin states are free to transfer about the crystal and these “moving spins” (spin entropy) carry energy which contributes to the power factor. This large unexpected power factor in layered cobalt oxide materials inspired the research for high ZT p-type materials in Ca3Co4O9 and Bi2Sr3Co2Oy structures. However, ZT is low due to high room temperature thermal conductivity of 4-5 W/m·K. It was later concluded that metal-oxide ZT values exceeding 2 would be limited by their large κT values, 3-10 W/m·K (compared with those of the heavy metallic alloys, 0.5-2 W/m·K). While these investigations have attracted a great deal of research, no major breakthroughs in oxide TE have yet emerged.